The instant invention relates generally to revolutionary new hydrogen storage alloys that are able, for the first time to realistically use the most ubiquitous, ultimate source of fuel, hydrogen. More specifically the instant invention relates to hydrogen storage alloys that not only are capable of storing on the order of 7 weight % hydrogen, but have extremely good absorption kinetics and have a long cycle life. A preferred method of producing the alloy is by gas atomization.
The instant patent application for describes hydrogen storage alloys, useful for a hydrogen-based economy, which have high storage capacity, excellent kinetics and long cycle life. An infrastructure system for such a hydrogen based economy, is the subject of copending U.S. application Ser. No. 09/444,810, entitled xe2x80x9cA Hydrogen-based Ecosystemxe2x80x9d filed on Nov. 22, 1999 (the ""810 application), which is hereby incorporated by reference. This infrastructure, in turn, is made possible by alloys such as the instant hydrogen storage alloy that have surmounted the chemical, physical, electronic and catalytic barriers that have heretofore been considered insoluble. Other hydrogen storage alloys which are useful in such an infrastructure are fully described in copending U.S. patent application Ser. No. 09/435,497, entitled xe2x80x9cHigh Storage Capacity Alloys Enabling a Hydrogen-based Ecosystemxe2x80x9d, filed on Nov. 6, 1999 (xe2x80x9cthe ""497 applicationxe2x80x9d), which is hereby incorporated by reference. The ""497 application relates to alloys which solve the unanswered problem of having sufficient hydrogen storage capacity with exceptionally fast kinetics to permit the safe and efficient storage of hydrogen to provide fuel for a hydrogen based economy, such as powering internal combustion engine and fuel cell vehicles. In the ""497 application the inventors for the first time disclosed the production of Mg-based alloys having both hydrogen storage capacities higher than about 6 wt. % and extraordinary kinetics. This revolutionary breakthrough was made possible by considering the materials as a system and thereby utilizing chemical modifiers and the principles of disorder and local order, pioneered by Stanford R. Ovshinsky, in such a way as to provide the necessary catalytic local environments, and at the same time designing bulk characteristics for storage and high rate charge/discharge cycling. In other words, these principles allowed for tailoring of the material by controlling the particle and grain size, topology, surface states, catalytic activity, microstructure, and total interactive environments for extraordinary storage capacity.
The combination of the ""810 and the ""497 applications solves the twin basic barriers which have held back the ubiquitous use of hydrogen: 1) storage capacity; and 2) infrastructure. With the use of the alloys of the ""497 application, hydrogen can be shipped safely by boats, barges, trains, trucks, etc. when in solid form. However, the hydrogen infrastructure described in the ""810 application requires careful thermal management and efficient heat utilization throughout the entire system. The instant invention makes the necessary heat transfer between the subsystems of the infrastructure simple, efficient, and economic.
As the world""s population expands and its economy increases, the atmospheric concentrations of carbon dioxide are warming the earth causing climate change. However, the global energy system is moving steadily away from the carbon-rich fuels whose combustion produces the harmful gas. For nearly a century and a half, fuels with high amounts of carbon have progressively been replaced by those containing less.
In the United States, it is estimated, that the trend toward lower-carbon fuels combined with greater energy efficiency has, since 1950, reduced by about half the amount of carbon spewed out for each unit of economic production. Thus, the decarbonization of the energy system is the single most important fact to emerge from the last 20 years of analysis of the system. It had been predicted that this evolution will produce a carbon-free energy system by the end of the 21st century. The instant invention helps to greatly shorten that period. In the near term, hydrogen will be used in fuel cells for cars, trucks and industrial plants, just as it already provides power for orbiting spacecraft. But ultimately, hydrogen will also provide a general carbon-free fuel to cover all fuel needs.
FIG. 1, taken from reliable industrial sources, is a graph demonstrating society""s move toward a carbon-free environment as a function of time starting with the use of wood in the early 1800s and ending in about 2010 with the beginning of axe2x80x9chydrogenxe2x80x9d economy. In the 1800s, fuel was primarily wood in which the ratio of hydrogen to carbon was about 0.1. As society switched to the use of coal and oil, the ratio of hydrogen to carbon increased first to 1.3 and then to 2. Currently, society is inching closer to the use of methane in which the hydrogen to carbon ratio is further increased to 4 (methane has serious problems with safety, cost and infrastructure). However, the ultimate goal for society is to employ a carbon-free fuel, i.e., the most ubiquitous of elements, pure hydrogen. The obstacle has been the lack of solid state storage capacity and infrastructure. The inventors of the ""497 and the ""810 applications have made this possible by inventing a 7% storage material (7% is an umoptimized fugure and will be increased along with better kinetics) with exceptional absorption/desorption kinetics, i.e. at least 80% charge in less than 2 minutes and an infrastructure to use these storage alloys. These alloys allow for the first time, a safe, high capacity means of storing, transporting and delivering pure hydrogen.
Hydrogen is the xe2x80x9cultimate fuel.xe2x80x9d It is inexhaustible. Hydrogen is the most plentiful element in the universe (over 95% of all matter). Hydrogen can provide a clean source of energy for our planet and can be produced by various processes which split water into hydrogen and oxygen. The hydrogen can then be stored and transported in solid state form.
While the world""s oil reserves are depletable, the supply of hydrogen remains virtually unlimited. Hydrogen, which can be produced from coal, natural gas and other hydrocarbons, is preferably formed via electrolysis of water, more preferably using energy from the sun (see U.S. Pat. No. 4,678,679, the disclosure of which is incorporated herein by reference.) However, hydrogen can also be produced by the electrolysis of water using any other form of economical energy (e.g., wind, waves, geothermal, hydroelectric, nuclear, etc.) Furthermore, hydrogen, is an inherently low cost fuel. Hydrogen has the highest density of energy per unit weight of any chemical fuel and is essentially non-polluting since the main by-product of xe2x80x9cburningxe2x80x9d hydrogen is water. Thus, hydrogen can be a means of solving many of the world""s energy related problems, such as climate change, pollution, strategic dependancy on oil, etc., as well as providing a means of helping developing nations.
While hydrogen has wide potential application as a fuel, a major drawback in its utilization, especially in mobile uses such as the powering of vehicles, has been the lack of an acceptable lightweight hydrogen storage medium. Storage of hydrogen as a compressed gas involves the use of large and heavy vessels. Additionally, large and very expensive compressors are required to store hydrogen as a compressed gas and compressed hydrogen gas is a very great explosion/fire hazard.
Hydrogen also can be stored as a liquid. Storage as a liquid, however, presents a serious safety problem when used as a fuel for motor vehicles since hydrogen is extremely flammable. Liquid hydrogen also must be kept extremely cold, below xe2x88x92253xc2x0 C., and is highly volatile if spilled. Moreover, liquid hydrogen is expensive to produce and the energy necessary for the liquefaction process is a major fraction of the energy that can be generated by burning the hydrogen. Another drawback to storage as a liquid is the costly losses of hydrogen due to evaporation, which can be as high as 5% per day.
For the first time, storage of hydrogen as a solid hydride, using the atomically engineered alloys of the ""497 application can provide a greater percent weight storage than storage as a compressed gas or a liquid in pressure tanks. Also, hydrogen storage in a solid hydride is safe and does not present any of the hazard problems that hydrogen stored in containers as a gas or a liquid does, because hydrogen, when stored in a solid hydride form, exists in it""s lowest free energy state.
In addition to the problems associated with storage of gaseous or liquid hydrogen, there are also problems associated with the transport of hydrogen in such forms. For instance transport of liquid hydrogen will require super-insulated tanks, which will be heavy and bulky and will be susceptible to rupturing and explosion. Also, a portion of the liquid hydrogen will be required to remain in the tanks at all times to avoid heating-up and cooling down of the tank which would incur big thermal losses. As for gaseous hydrogen transportation, pressurized tankers could be used for smaller quantities of hydrogen, but these too will be susceptible to rupturing and explosion. For larger quantities, a whole new hydrogen pipeline transportation system would need to be constructed or the compressor stations, valves and gaskets of the existing pipeline systems for natural gas will have to be adapted and retrofitted to hydrogen use. This assumes, of course, that the construction material of these existing pipelines will be suited to hydrogen transportation.
A high hydrogen storage capacity per unit weight of material is an important consideration in applications where the hydride does not remain stationary. A low hydrogen storage capacity relative to the weight of the material reduces the mileage and hence the range of the vehicle making the use of such materials impractical. A low desorption temperature (in the neighborhood of 300xc2x0 C.) is desirable to reduce the amount of energy required to release the hydrogen. Furthermore, a relatively low desorption temperature to release the stored hydrogen is necessary for efficient utilization of the available exhaust heat from vehicles, machinery, or other similar equipment.
Good reversibility is needed to enable the hydrogen storage material to be capable of repeated absorption-desorption cycles without significant loss of its hydrogen storage capabilities. Good kinetics are necessary to enable hydrogen to be absorbed or desorbed in a relatively short period of time. Resistance to poisons to which the material may be subjected during manufacturing and utilization is required to prevent a degradation of acceptable performance.
The prior art metallic host hydrogen storage materials include magnesium, magnesium nickel, vanadium, iron-titanium, lanthanum pentanickel and alloys of these metals others. No prior art material, however, has solved the aforementioned problem which would make it suitable for a storage medium with widespread commercial utilization which can revolutionize the propulsion industry and make hydrogen a ubiquitous fuel.
Thus, while many metal hydride systems have been proposed, the Mg systems have been heavily studied since Mg can store over 7 weight % of hydrogen. While magnesium can store large amounts of hydrogen, its has the disadvantage of extremely slow kinetics. For example, magnesium hydride is theoretically capable of storing hydrogen at approximately 7.6% by weight computed using the formula: percent storage=H/H+M, where H is the weight of the hydrogen stored and M is the weight of the material to store the hydrogen (all storage percentages hereinafter referred to are computed based on this formula). Unfortunately, despite high storage capacity, prior art materials were useless because discharge of the hydrogen took days. While a 7.6% storage capacity is ideally suited for on board hydrogen storage for use in powering vehicles, it requires the instant invention to form Mg-based alloys operating on principles of disorder to alter previously unuseable materials and make them commercially acceptable for widespread use.
Magnesium is very difficult to activate. For example, U.S. Pat. No. 3,479,165 discloses that it is necessary to activate magnesium to eliminate surface barriers at temperatures of 400xc2x0 C. to 425xc2x0 C. and 1000 psi for several days to obtain a reasonable (90%) conversion to the hydride state. Furthermore, desorption of such hydrides typically requires heating to relatively high temperatures before hydrogen desorption begins. The aforementioned patent states that the MgH2 material must be heated to a temperature of 277xc2x0 C. before desorption initiates, and significantly higher temperatures and times are required to reach an acceptable operating output. Even then, the kinetics of pure Mg are unacceptable, i.e., unuseable. The high desorption temperature makes the prior art magnesium hydride unsuitable.
Mg-based alloys have been considered for hydrogen storage also. The two main Mg alloy crystal structures investigated have been the A2B and AB2 alloy systems. In the A2B system, Mg2Ni alloys have been heavily studied because of their moderate hydrogen storage capacity, and lower heat of formation (""64 kJ/mol)than Mg. However, because Mg2Ni has the possibility of a storage capacity of up to 3.6 wt. % hydrogen, researchers have attempted to improve the hydrogenation properties of these alloys through mechanical alloying, mechanical grinding and elemental substitutions. However, 3.6 wt. % is not nearly high enough and the kinetics are likewise insufficient.
More recently, investigators have attempted to form MgNi2 type alloys for use in hydrogen storage. See Tsushio et al, Hydrogenation Properties of Mg-based Laves Phase Alloys, Journal of Alloys and Compounds, 269 (1998), 219-223. Tsushi et al. determined that no hydrides of these alloys have been reported, and they did not succeed in modifying MgNi2 alloys to form hydrogen storage materials.
Finally, the instant inventors have worked on high Mg content alloys or elementally modified Mg. For instance, in U.S. Pat. Nos. 5,976,276 and 5,916,381, Sapru, et al have produced mechanically alloyed Mgxe2x80x94Nixe2x80x94Mo and Mgxe2x80x94Fexe2x80x94Ti materials containing about 75 to 95 atomic percent Mg, for thermal storage of hydrogen. These alloys are formed by mixing the elemental ingredients in the proper proportions in a ball mill or attritor and mechanically alloying the materials for a number of hours to provide the mechanical alloy. While these alloys have improved storage capacities as compared with Mg2Ni alloys, they have low plateau pressures.
Another example of modified high Mg content alloy is disclosed in U.S. Pat. No. 4,431,561 (""561) to Ovshinsky et al., the disclosure of which is hereby incorporated by reference. In the ""561 patent, thin films of high Mg content hydrogen storage alloys were produced by sputtering. While this work was remarkable in applying fundamental principles to drastically improve the storage capacities, it was not until the invention described herein that all necessary properties of high storage capacity, good kinetics and good cycle life were brought together.
In U.S. Pat. No. 4,623,597 (xe2x80x9cthe ""597 patentxe2x80x9d), the disclosure of which is incorporated by reference, one of the present inventors, Ovshinsky, described disordered multicomponent hydrogen storage materials for use as negative electrodes in electrochemical cells for the first time. In this patent, Ovshinsky describes how disordered materials can be tailor-made to greatly increase hydrogen storage and reversibility characteristics. Such disordered materials are formed of one or more of amorphous, microcrystalline, intermediate range order, or polycrystalline (lacking long range compositional order) wherein the polycrystalline material may include one or more of topological, compositional, translational, and positional modification and disorder, which can be designed into the material. The framework of active materials of these disordered materials consist of a host matrix of one or more elements and modifiers incorporated into this host matrix. The modifiers enhance the disorder of the resulting materials and thus create a greater number and spectrum of catalytically active sites and hydrogen storage sites.
The disordered electrode materials of the ""597 patent were formed from lightweight, low cost elements by any number of techniques, which assured formation of primarily non-equilibrium metastable phases resulting in the high energy and power densities and low cost. The resulting low cost, high energy density disordered material allowed such Ovonic batteries to be utilized most advantageously as secondary batteries, but also as primary batteries and are used today worldwide under license from the assignee of the subject invention.
Tailoring of the local structural and chemical order of the materials of the ""597 patent was of great importance to achieve the desired characteristics. The improved characteristics of the anodes of the ""597 patent were accomplished by manipulating the local chemical order and hence the local structural order by the incorporation of selected modifier elements into a host matrix to create a desired disordered material. The disordered material had the desired electronic configurations which resulted in a large number of active sites. The nature and number of storage sites was designed independently from the catalytically active sites.
Multiorbital modifiers, for example transition elements, provided a greatly increased number of storage sites due to various bonding configurations available, thus resulting in an increase in energy density. The technique of modification especially provides non-equilibrium materials having varying degrees of disorder provided unique bonding configurations, orbital overlap and hence a spectrum of bonding sites. Due to the different degrees of orbital overlap and the disordered structure, an insignificant amount of structural rearrangement occurs during charge/discharge cycles or rest periods therebetween resulting in long cycle and shelf life.
The improved battery of the ""597 patent included electrode materials having tailor-made local chemical environments which were designed to yield high electrochemical charging and discharging efficiency and high electrical charge output. The manipulation of the local chemical environment of the materials was made possible by utilization of a host matrix which could, in accordance with the ""597 patent, be chemically modified with other elements to create a greatly increased density of catalytically active sites for hydrogen dissociation and also of hydrogen storage sites.
The disordered materials of the ""597 patent were designed to have unusual electronic configurations, which resulted from the varying 3-dimensional interactions of constituent atoms and their various orbitals. The disorder came from compositional, positional and translational relationships of atoms. Selected elements were utilized to further modify the disorder by their interaction with these orbitals so as to create the desired local chemical environments.
The disorder described in the ""597 patent can be of an atomic nature in the form of compositional or configurational disorder provided throughout the bulk of the material or in numerous regions of the material. The disorder also can be introduced into the host matrix by creating microscopic phases within the material which mimic the compositional or configurational disorder at the atomic level by virtue of the relationship of one phase to another. For example, disordered materials can be created by introducing microscopic regions of a different kind or kinds of crystalline phases, or by introducing regions of an amorphous phase or phases, or by introducing regions of an amorphous phase or phases in addition to regions of a crystalline phase or phases. The interfaces between these various phases can provide surfaces which are rich in local chemical environments which provide numerous desirable sites for electrochemical hydrogen storage.
The differences between chemical and thermal hydrides are fundamental. The thermal hydride alloys of the present inventions have been designed as a distinct class of materials with their own basic problems to be solved, which problems as shown in the following Table 1 are antithetical to those to be solved for electrochemical systems.
These same attributes have not, until now, been achieved for thermal hydrogen storage alloys. Therefore, there has been a strong felt need in the art for high capacity, low cost, light weight thermal hydrogen storage alloy materials having exceptionally fast kinetics.
As alluded to above, cycle life of a hydrogen storage alloy material is very important for commercial utilization of the alloy. Cycle life of powdered Mg has been, prior to the subject invention, another one of its deficiencies. Specifically, it has been noted in the prior art that even Mg powder loses it""s capacity and kinetics during hydriding/dehydriding cycling after only 40 cycles or so. This loss of capacity and kinetics is due to sintering of the Mg particles during the required high temperature cycling, and the formation of a large mass of hydrogen. For commercial applications this is not nearly long enough and at least an order of magnitude longer cycle life is minimally needed. Thus there is a strong felt need in the art for a hydrogen storage alloy which has a high storage capacity, good kinetics and a long cycle life.
Disclosed herein is a magnesium-based hydrogen storage alloy powder. The alloy has a high hydrogen storage capacity, fast hydrogen adsorption kinetics and a long cycle life. The alloy is characterized in that it has an intergranular phase which prevents sintering of the alloy particles during high temperature hydriding/dehydriding thereof, thus allowing for a long cycle life. The magnesium-based hydrogen storage alloy powder comprises at least 90 weight % magnesium, and has: a) a hydrogen storage capacity of at least 6 weight % (preferably at least 6.9 wt %); b) absorption kinetics such that the alloy powder absorbs 80% of it""s total capacity within 5 minutes at 300xc2x0 C. (preferably within 1.5 minutes); and c) a particle size range of between 30 and 70 microns. The alloy also includes Ni and Mm (misch metal) and can also include additional elements such as Al, Y, B, C and Si. Thus the alloys will typically contain 0.5-2.5 weight % nickel and about 1.0-5.5 weight % Mm (predominantly contains Ce, La, Pr and Nd). The alloy may also contain one or more of: 3-7 weight % Al; 0.1-1.5 weight % Y; 0.1-3.0 weight % B; 0.1-3.0 weight % C; and 0.3-2.5 weight % silicon. The alloy is preferably produced via atomization (such as inert gas atomization), a rapid solidification process in which the quench rate is controlled to be between 103-104xc2x0 C./s.
The magnesium based hydrogen storage alloy powder has a microstructure which includes at least two distinct regions: 1) a major hydrogen storage phase, surrounded by 2) an intergranular material region. The intergranular material is enriched in mischmetal, silicon and nickel and reduced in magnesium as compared with said hydrogen storage phase. Thus, the intergranular material (once hydrided) has a higher melting point than the storage phase, and the permanent hydride formed in the intregranular region inhibits sintering of the alloy particles during hydride/dehydride cycling thereof. This unique microstructure is formed by cooling the alloy from a melt at a cooling rate of about 103-104xc2x0 C.